Sample spec for https://twitter.com/ms2sato/status/623787172999507969

mock_spec.rb

# see also 
# https://relishapp.com/rspec/rspec-mocks/v/3-9/docs/configuring-responses/block-implementation#simulating-a-transient-network-failure
describe 'Mock' do
  specify do
    foo = double
    counter = 0
    allow(foo).to receive(:bar) do
      if counter.zero?
        counter += 1
        'abc'
      else
        raise 'This is test.'
      end
    end
    expect(foo.bar).to eq 'abc'
    expect{foo.bar}.to raise_error("This is test.")
  end
end